The golf club/ball impact can best be described as a violent collision. The typical professional can swing a 200 gram (7.06 ounces) to 300 gram (10.6 ounces) driver and attain club speeds at the moment of impact of 105 to 115 mph, striking a 46 gram (1.62 ounces) golf ball resting on a tee. One side of the golf ball is struck with a golf club which can result in the balls of the prior art compressing nearly 50% before the golf ball leaves the tee. The golf ball then accelerates from rest to speeds of approximately 230 ft/s (70 m/s) to 240 ft/s (73 m/s) and spin rates of 2000 to 4000 rpm's in less than half a millisecond, experiencing 50,000 times the force of gravity.
For a great number of years, golf balls were molded using wound cores, which comprised a soft rubber center surrounded by a layer of thread rubber windings. In the late 1960s to early 1970s, balls with ionomer covers (produced by E.I. du Pont de Nemours and Company, 1007 Market ST Wilmington, Del. 19898 (“DuPont”) under the trade name Surlyn®) were introduced. Balls molded with Surlyn® covers were produced with both thread wound cores and solid rubber cores. The balls molded using initial grades of Surlyn® and solid cores (hereafter referred to as “two-piece balls”) were significantly less expensive to produce; however, the initial two-piece golf balls were hard, having an unpleasant feel to the golfer.
In the late 1980s, DuPont came out with softer Surlyn® terpolymer grades, known as Very Low Modulus Ionomers (V.L.M.I.). These materials allowed for development of two-piece golf balls with softer covers; however, use of high levels of V.L.M.I. results in a significant detrimental effect on the golf ball resilience. The limitation on balls made with V.L.M.I. materials was (is) that use of high levels of V.L.M.I. materials has a significant detrimental effect on golf ball resilience properties. Therefore, golf balls with soft covers could be made, but had relatively high compression; thus exhibiting high spin rates and low velocity.
In the mid- to late-1990s, softer, i.e. lower compression, distance type golf balls were developed. These golf balls included the addition of an intermediate cover layer. The additional layer allowed for greater control of the performance properties of the golf ball. In the late 1990's, multi-layer golf balls utilizing polyurethane outer covers were introduced. These balls where rapidly adopted by professional golfers due to their premium qualities. However, these balls required a hard feel to achieve the desired distance and spin properties.
Through a softer core, a golf ball molded with a stiff ionomer had a reasonable feel based upon a relatively low compression; however, the core compression can only be reduced to a certain level (a Professional Golfers Association (PGA) compression of about 35) while retaining acceptable ball durability. If a core compression of below about 35 was used, impact durability of the golf ball was poor. A favorable byproduct of the use of a soft compression core in a golf ball was a lower spin rate, which allowed for better accuracy of the golf ball.
In 1998, Wilson Sporting Goods Co. (“Wilson”), 8700 West Bryn Mawr Avenue, Chicago, Ill. 60631, introduced a golf ball made using a core with about a 35 compression (sold under the trademark Staff® Titanium Straight Distance. To keep the velocity and performance properties of a premium distance golf ball, Wilson used a stiff ionomer cover layer on this ball. The ball compression of this golf ball was approximately 85, which was low for the time when it was introduced.
Existing golf balls, however, have some drawbacks. Prior art golf balls are generally manufactured with a core made primarily from polybutadiene rubber, which is covered with a fairly hard, thin, ionomer inner cover layer, which is subsequently covered by the polyurethane or balata/polybutadiene outer cover layer. While providing adequate playing characteristics at a less expensive production cost, these solid balls exhibit lower velocities at driver impact than wound balls using like cover materials. Prior art golf balls utilized either thermoplastic or thermoset material for the covers. The prior art thermoplastic material allows for greater ease in manufacturing, but reduces resilience. Conversely, thermoset material is difficult with which to work, but provides needed resilience.
In addition, all of the various materials used in the construction of golf balls, from wound core constructions through to multi-layer solid core constructions, have varying densities. Accordingly, the mass per unit volume of these materials varies. For example, typically, the materials used to produce the cover layer possess a lower mass per unit volume than the materials used to produce the core. Additionally, the material composition of most intermediate layers has a density or a weight per unit volume that is different than the density or weight per unit volume of the core and/or the cover layer. If a golf ball is manufactured perfectly, that is if the core or center of a ball is perfectly spherical, and if the cover layer thickness and intermediate layer thickness (if applicable) are constant throughout the entire ball, the ball will be “balanced”, and should fly true when struck with a golf club, or should roll true when putted.
More recently, golf balls have been developed with significantly lower ball compression than was previously considered possible for a premium two-piece golf ball. The Precept® Lady and Laddie golf balls (produced by Bridgestone Sports Co., LTD., Omori Bellport E Bldg. 6-22-7, Minami-oi Shinagawa-ku, Tokyo 140-0013 Japan), and the Titleist® NXT Distance golf balls (produced by Fortune Brands, Inc., 300 Tower Parkway, Lincolnshire, Ill. 60069), were introduced in the early 2000s. These golf balls have compressions ranging from about 55 (the Precept® Laddie) to the mid to upper 60s (Titleist® NXT Distance and Precept® Lady). These balls are designed to produce low ball compression through the use of softer cover materials (ionomer blends comprising varying levels of V.L.M.I. materials). However, existing golf ball cores are not formed with a compression value below 35. Further, the lowest compression golf ball currently made that performs with properties acceptable for a premium golf ball is the Precept® Laddie, with a compression in the low 50s.
Thus, there exists a need for a golf ball which has a low compression and maintains the performance and properties expected from a premium golf ball, such as, but not limited to low spin rate, good feel and good impact durability.